The present invention relates to a process for manufacturing a preform obtained by injecting into a mould at least one thermoplastic resin, in a substantially crystalline state, as well as to an installation for implementing this process. It also relates to the plastic recipient obtained by drawing and blowing in a mould a preform manufactured according to the process of the invention. The invention resides more precisely in an improvement made to the operation of drying the resin in the form of granules, which drying operation is carried out prior to injecting the resin into the mould. The invention will primarily, but not exclusively, find an application in the manufacture of plastic preforms and recipients, based on at least one polyethylene terephthalate resin (PET), that have a low acetaldehyde (AA) content, which makes them primarily, although not exclusively, suitable for storing mineral water.
In the packing industry, and more especially in the field of bottling, the use of plastic recipients that are obtained by drawing and blowing a preform in a mould is very widespread. Usually, the preform is obtained by injecting one or more thermoplastic resins into a mould. Those thermoplastic resins most commonly used to date, on account of their low cost price, are polyethylene terephthalate (PET) homopolymer or copolymer based resins. These PET resins have, however, the major drawback of being permeable to gases, in particular to oxygen and carbon dioxide. When it is wished to obtain recipients having specific functional properties that cannot be obtained with PET resins, more specific thermoplastic resins having the properties sought after are used to manufacture the preform. For example, when it is wished to produce a plastic recipient having improved gas barrier properties, it is known to use specific thermoplastic resins such as polyamide resins containing m-xylylene groups and commonly referred to as xe2x80x9cMX-nylonxe2x80x9d, resins based on a vinyl alcohol and ethylene copolymer and, for example, EVOH resins, or again, polyethylene 2,6-naphthalene dicarboxylate (PEN) resins, etc. These specific resins can be used by being mixed, prior to injection, with a PET or similar resin, the mixture obtained usually being injected into a mould to produce the preform. More commonly, these specific resins are used for the manufacture of preforms having a multilayer structure, by sequential and/or parallel co-injection of a specific resin and of a PET or similar resin.
Usually, the thermoplastic resin or resins, which are used to manufacture a preform, have a degree of crystallisation greater than 50% and initially take the form of solid granules. These granules, which are in a substantially crystalline state, are obtained in a known manner by subjecting granules of resin in an amorphous state to a heat treatment in the granule manufacturing step. Granules of resin in an amorphous state have the drawback of tending to stick together and form agglomerates. This preliminary step of crystallisation of these resin granules, which is implemented during the granule manufacturing step, advantageously makes it possible to obtain resin granules which, owing to their crystalline state, no longer tend to agglomerate together. One example of a process of crystallisation of polyester resin granules is described, in particular, in European patent application EP-A-0 379 684. In this publication, the polyester resin granules in a crystalline state are obtained by subjecting polyester resin granules in an amorphous state to a heat treatment taking place in two successive steps.
To manufacture a preform from a thermoplastic resin substantially in a crystalline state and in the form of granules, the following successive operations are performed:
melting and mixing the resin granules by passing them through a screw type extruder;
injecting the melted resin from the extruder into a mould.
As a rule, the aforementioned step of melting and mixing the granules of resin at the time of manufacturing a preform is preceded by a step of storing these resin granules in a substantially crystalline state, which may vary in length, depending on the particular case. The thermoplastic resins most commonly used to manufacture the preforms and, in particular, PET resins, absorb the moisture of the atmosphere when stored in the form of granules for a certain time. This moisture present in the resin, because of the high temperatures used to produce the melting of the resin granules and to inject the melted resin into the preform mould, generates degradation through hydrolysis of the resin. This degradation by hydrolysis is all the more marked the greater the moisture content of the resin and/or the higher the resin heating temperatures. This hydrolytic degradation of the resin undesirably results in a reduction in the weight of the molecule of the resin, thus reducing the intrinsic viscosity of the resin and the associated properties. Now, it is known that an excessive loss of intrinsic viscosity can impair the quality of the preform obtained and, in particular, can impair the transparency of the preform and can lead to a loss of the mechanical properties of the bottle obtained from the preform. It is thus essential for preform manufacturers to reduce this phenomenon of degradation through hydrolysis of the resin as far as possible. For this purpose, prior to transformation of the resin by passing through the extruder screw, the granules of resin in a crystalline state are dried in a preliminary operation, which makes it possible to reduce the relative moisture content of the resin granules. To date, this drying operation has been carried out by passing the resin granules through a drying hopper, the output of which is connected to the input of the extruder, and which delimits an internal drying chamber brought to a given temperature, by means of a flow of dry, hot air that is continually renewed.
During the process of manufacturing a preform, the resin or resins used also undergo thermal degradation, which occurs chiefly when the resin passes through the screw extruder. This thermal degradation is harmful as it leads, on one hand, to a loss of the physical properties of the resin, reflected, in particular, in a loss of intrinsic viscosity, the appearance of traces of crystallinity on the preform or again, a change in the colour of the preforms and, on the other hand, to the formation of undesirable by-products. In particular, in the case of a PET resin, one of the undesirable by-products resulting from the thermal degradation of the resin is acetaldehyde (CH3CHO), which is produced chiefly in gaseous form, and which is characterized by a fruity smell and taste. The acetaldehyde ends up in the walls of the recipient that is produced from the preform, and it migrates, during storage, in contact with the product stored in the recipient. If the acetaldehyde content is too high, the result is a substantial deterioration in the taste of the product stored, a deterioration in taste that can be undesirably perceived by the consumer. In the case of a PET resin, it is thus important to ensure that as little acetaldehyde as possible is generated. It is vital to maintain a very low acetaldehyde content when the product stored in the recipient is one with little taste, such as, for example, mineral water. This is why, in the particular field of mineral water storage, bottlers impose on their suppliers of PET based bottles a maximum acetaldehyde content that must not be exceeded.
One of the difficulties of the operation of drying granules of resin in a crystalline state lies in the choice of temperature and the duration of drying in the hopper. To ensure that the water absorbed by the resin is efficiently removed, and to bring the relative moisture content of the resin down to acceptable levels, the temperature and the drying time have to be sufficient. Conversely, the temperature and the drying time must be sufficiently reduced to limit as far as possible any thermal and hydrolytic degradation of the resin at the stage of the drying operation. In the case of PET resins, for example, it is now known that the hydrolysis of these resins in a solid state can begin at a temperature of 150xc2x0 C., admittedly at a slow rate, but one that depends on the temperature and increases as the temperature rises. This is why it is preferred, to date, to limit the drying temperature for PET resins to levels such that the resin at the output from the drying hopper is at a temperature of less that 150xc2x0 C. As a rule, the drying air introduced into the hopper is brought to a temperature in the order of 140xc2x0 C., which corresponds to a resin temperature in the order of 135xc2x0 C., and the drying time for PET resins is generally between 6 and 8 hours.
The invention provides a process for manufacturing a preform from at least one thermoplastic resin, substantially in a crystalline state, the step of drying which has been improved. In the present text, a resin xe2x80x9csubstantiallyxe2x80x9d in a crystalline state is to be taken as meaning a resin having a degree of crystallinity higher than 50%.
As known to date, the manufacturing process according to the invention includes the following successive operations:
drying the thermoplastic resin substantially in a crystalline state and in the form of granules;
melting the dried granules of resins;
injecting the melted resin into a mould and removing the preform from the mould.
In a characteristic, novel manner according to the invention, the drying operation is carried out in at least two successive separate steps.
in a first step, the granules of resin substantially in a crystalline state are dried for a predetermined duration (t1) at a predetermined drying temperature (T1);
in a second step, the drying of the granules from the first step is continued at a drying temperature (T2) that is markedly higher than the drying temperature (T1) of the first stage, and for a duration (t2) that is markedly shorter than the period (t1) of the first step.
The advantage of dividing the drying operation into at least two separate steps is that it permits better optimisation of the resin drying process. During the resin drying process, the risk of its hydrolytic degradation is, in fact, maximum at the start of drying, when the relative moisture content of the resin is highest, and it declines as the drying process progresses, the relative moisture content of the resin gradually decreasing. Conversely, the risk of thermal degradation of the resin is smaller, even non-existent, at the beginning of the drying process, but increases during the drying process owing to the reduction of the resin""s relative moisture content. Thanks to the process according to the invention, as, at the time of the first drying-step, the risks of hydrolytic degradation predominate, a drying temperature is chosen that is sufficiently low to preclude, or at the very least limit, hydrolytic degradation of the resin. At the end of the first step, the relative moisture content of the resin having dropped, it becomes advantageously possible to operate at a higher drying temperature, without any risk of major hydrolytic degradation of the resin. The duration of this second step in the drying operation must, however, be sufficiently short to preclude, or at the very least limit, thermal degradation of the resin, which is predominant at this stage in the drying process in relation to hydrolytic degradation.
Thanks to the process according to the invention, it becomes advantageously possible to operate with a drying temperature (the temperature of the second drying step) that is higher than the drying temperatures used to date, and without harm in terms of thermal and hydrolytic degradation of the resin during the drying operation. The main result of this is that one can obtain, at the end of the drying operation, dried granules of resin the temperature of which is higher than with the prior art is process. Comparatively speaking, if, using the prior art process, one attempted to apply a higher drying temperature, identical with the drying temperature used in the second step of the process according to the invention, more extensive hydrolytic degradation of the resin would be undesirably caused during the drying operation than in the process according to the invention, as well as thermal degradation that would be at least comparable with that of the process according to the invention and, in all probability, more extensive.
Thanks to the invention, by obtaining a higher resin temperature at the exit from the drying operation, the following resin melting and mixing operation advantageously requires less thermal and mechanical energy. Incidentally, it becomes possible to contemplate reducing the resin heating temperature, that is to say, in practice, the temperature of the screw extruder, without any deterioration in the resin melting process, which makes it possible to reduce thermal degradation of the resin when it is transformed and, through this very fact, to reduce the production of undesirable degradation by-products in the course of the preform manufacturing process. It is for the person skilled in the art to fix the parameters (t1, T1, t2, T2) of the process according to the invention, on a case by case basis, according to the installation used for manufacturing the preforms and to the type of thermoplastic resin used.
The invention also relates to the plastic recipient obtained by drawing and blowing in a mould a preform manufactured according to the aforementioned process, as well as to an installation for implementing this process.
The installation according to the invention usually comprises means for drying the thermoplastic resin in the form of granules and in a substantially crystalline state, the output of which is connected to the input of an extruder, which extruder feeds, at its output, an injection mould.
Characteristically, according to the invention, the drying means comprise:
at least two successive drying hoppers, namely a first drying hopper provided for processing one volume of resin (V1) and a second drying hopper provided for processing a volume of resin (V2) that is markedly smaller then the volume (V1) processed by the first hopper;
means enabling the first drying hopper to be fed with the thermoplastic resin in the form of granules;
means enabling the granules of resin from the first hopper to be directed to the input of the second hopper;
means enabling a drying gas to be circulated inside the first drying hopper, and a drying gas to be circulated inside the second drying hopper;
first heating means designed to bring the drying gas, introduced into the first hopper, to a drying temperature (T1); and
second heating means designed to bring the drying gas introduced into the second hopper to a drying temperature (T2) that is markedly higher than the drying temperature (T1) of the first heating means.